V=-N(dΦ/dt)
Electricity and Magnetism

Electromagnetic induction

for 14-16

Generators and motors are the workhorses of modern industrial society. 
 
The experiments in this collection should be seen as a continuous development of ideas about electromagnetic induction.

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The motor as a dynamo

V=-N(dΦ/dt)
Electricity and Magnetism

The motor as a dynamo

Practical Activity for 14-16

Class practical

This simple experiment shows the reversibility of motor and dynamo effects.

Apparatus and Materials

For each student group

  • Electric motor, small e.g. one constructed by student group in earlier activity
  • Galvanometer, sensitive to e.g. 3.5–0–3.5 mA, 10 ohm resistance (see note below)
  • Crocodile clips, 2
  • Leads, 4 mm, 2
  • Copper wire, insulated with bare ends, 200 cm

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

Choose electric motors which have an axle which can be readily turned by hand. They must also have two contacts where the crocodile clips can be attached.

Read our standard health & safety guidance


See the experiment:

The electric motor


Procedure

  1. Connect the electric motor to the galvanometer.
  2. Spin the motor; observe the galvanometer. You can spin the motor with a finger and thumb on the axle; alternatively, wrap a length of cotton once round the axle and pull from either end, as illustrated.
  3. Investigate the factors which affect the galvanometer reading. What happens if you reverse the direction of spin?
  4. If you are using a model electric motor, you can convert it so that it will generate alternating current (AC) as follows.
  5. Remove the rubber bands and undo the commutator wiring.
  6. Insulate both ends of the aluminium tube by wrapping adhesive tape around them.
  7. Bring out the ends of the leads at opposite ends of the armature.
  8. Bare the leads for two or three centimetres, and wind the bare ends tightly around the aluminium tube.
  9. Make a brush at each end and connect up to the meter.
  10. Investigate the factors which affect the galvanometer reading. What happens if you reverse the direction of spin now?

Teaching Notes

  • Turning a motor by hand makes the motor into a dynamo. The potential difference can be measured on a sensitive galvanometer. Frequently it is enough just to use a sensitive ammeter, as long as confusion does not arise. The dynamo effect produces an e.m.f. (a potential difference}, and not a current.
  • Students should note that turning the dynamo faster produces a greater deflection of the meter. Reversing the spin of the dynamo generates a deflection in the opposite direction.
  • The wave form of this DC dynamo can be observed on a C.R.O. It will not be a flat graph but rather follow the changes in magnitude of the e.m.f. throughout the cycle. However, it will be uni-directional. If either brush fails to make a connection as it rotates, there will be a considerable AC potential difference between the input terminals of the C.R.O. This will cause a misleading trace of AC mains on the tube. Connecting a 10 kΩ resistor across the input terminals of the C.R.O. will prevent this.
  • If you use a home-made model electric motor which has run for an appreciable time, the brushes and commutator will be dirty and have a high resistance. Strip down and scrape the brushes and commutator with emery-paper to clean them. Avoid finger grease. Taking these precautions may increase the galvanometer deflection several times.
  • AC version: As an alternative to the slip rings, it is possible to make a temporary dynamo that will work for a few turns. Bring out a pair of leads of thin wire from the coil, and let the leads twist up as the coil is turned.
  • The AC wave form produced by the dynamo is best seen by connecting the dynamo to a C.R.O. The trace should resemble the traditional sine wave.

This experiment was safety-tested in July 2007

  • A video showing a similar electromagnetic induction practical:

Up next

Magnet and coil

V=-N(dΦ/dt)
Electricity and Magnetism

Magnet and coil

Practical Activity for 14-16

Class practical

An introduction to the dynamo principle.

Apparatus and Materials

For each student group

  • Permanent bar magnet
  • Copper wire, insulated with bare ends, 200 cm
  • Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Wind a coil of 10 to 20 turns with long leads (say 50 cm). The coils should be such that a permanent bar magnet can pass freely through.
  2. Connect the long leads to the galvanometer.
  3. Move the magnet in the space in and around the coil, keeping an eye on the galvanometer. Summarize your observations.

Teaching Notes

  • You might introduce this experiment by saying:
  • "A dynamo or generator is a carefully-designed piece of equipment. There is a coil of wire and a magnetic field. There is motion. Electricity (a voltage) is generated. You can understand the principle of the dynamo by starting with a simpler situation: you have a coil and a magnet, and you can move them. What will you discover?"
  • The students should find out that:
    • The current flows only when the magnet and the coil are moving relative to each other;
    • The current changes direction when the magnet is inserted into the coil and then removed from the coil;
    • More turns on the coil produce bigger currents provided the total length (i.e. the total resistance) of the wire remains the same;
    • The faster the magnet is moved, the greater is the maximum deflection.

This experiment was saftey-checked in April 2006

Up next

Cutting a magnetic field with a wire

V=-N(dΦ/dt)
Electricity and Magnetism

Cutting a magnetic field with a wire

Practical Activity for 14-16

Class practical

Students should previously have inserted a magnet into a coil and measured the e.m.f. generated. This is a simpler version showing first of all only one coil (or loop) of wire.

Apparatus and Materials

  • Mild steel yoke
  • Copper wire, insulated with bare ends, 200 cm
  • Magnadur magnets, 2
  • Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)-

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Attach two Magnadur magnets to the steel yoke with opposite poles facing each other.
  2. Connect a long lead of insulated copper wire to the galvanometer.
  3. Move the wire through the field between the permanent magnets.
  4. Try the effect of a coil of many turns (see picture) and see how this changes the deflection.

Teaching Notes

Students will find that:

  • There is only a current when the wire and magnet are moving relative to each other
  • The faster the magnet or wire is moved then the greater the current
  • The current changes direction when the relative motion of the wire and the magnetic field changes direction
  • The effect is greater when the wire is formed into a coil (because there is more wire moving across the magnetic field)

This experiment was safety-tested in April 2006

Up next

A magnet moving near a coil on a C-core

V=-N(dΦ/dt)
Electricity and Magnetism

A magnet moving near a coil on a C-core

Practical Activity for 14-16

Class practical

In this experiment the coil of the wire is wound on a soft iron core and a permanent magnet is moved relative to it.

Apparatus and Materials

For each student group

  • Copper wire, insulated with bare ends, 200 cm
  • C-core, laminated iron
  • Permanent bar magnet
  • Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Wind a coil of roughly 20 turns on one arm of a C-core.
  2. Connect the coil by long leads to a galvanometer.
  3. Place a magnet across the ends of the core. Observe the effect.
  4. Remove the magnet. Again observe the effect.
  5. Find out how the deflection on the galvanometer changes if the core is removed from the coil.
  6. Investigate the factors which affect the deflection on the galvanometer.

Teaching Notes

Students will find that:

  • There is only a current when the coil and magnet are moving relative to each other;
  • Faster movement results in a bigger deflection;
  • The iron core increases the size of the current;
  • The current changes direction when the magnet moves towards and away from the coil.

This experiment was safety-tested in December 2004

Up next

Moving an electromagnet

V=-N(dΦ/dt)
Electricity and Magnetism

Moving an electromagnet

Practical Activity for 14-16

Class practical

An electromagnet is now used instead of a permanent magnet with similar effects. This is to be expected, but it is satisfying to see in action.

Apparatus and Materials

  • Copper wire, insulated with bare ends, 200 cm
  • C-cores, laminated iron, 2
  • Cell, 1.5 V in holder
  • Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

If a zinc chloride cell is used, it will polarize in 60 s or less and must be left overnight to recover.

If an alkaline manganese cell is used, there is a danger of the cell overheating with a risk of explosion: complete the circuit for 30 s or less.

If a re-chargeable cell (NiCd) is used, the wire will get very hot and the cell will be discharged in a few minutes: do the experiment as quickly as possible.

Read our standard health & safety guidance


It is possible to use a low-voltage power supply instead of the 1.5 V cell, but any ripple on the d.c. output can lead to confusion.

Procedure

  1. Wind a coil of roughly 20 turns on one arm of a C-core.
  2. Connect the coil by long leads to a galvanometer. This is Coil 1.
  3. Wind a coil of 10 turns on one arm of the second C-core.
  4. Connect this coil to the 1.5 V cell. This is Coil 2.
  5. Coil 2 becomes an electromagnet. Bring it up to Coil 1, as shown. Observe the effect.
  6. Take Coil 2 away again. Observe the effect.
  7. Find out how the deflection on the galvanometer changes if the current in Coil 2 is reversed.
  8. Investigate the factors which affect the deflection on the galvanometer.

Teaching Notes

Students will find that:

  • There is only a current when Coils 1 and 2 are moving relative to each other;
  • Reversing the movement of Coil 2 reverses the deflection on the galvanometer;
  • Reversing the current of Coil 2 reverses the deflection on the galvanometer;
  • Faster movement results in a bigger deflection.

This experiment was safety-tested in January 2005

Up next

Switching an electromagnet

V=-N(dΦ/dt)
Electricity and Magnetism

Switching an electromagnet

Practical Activity for 14-16

Class practical

Now is the time to be lazy! Instead of moving the electromagnet just switch it on or off.

Apparatus and Materials

  • C-cores, laminated iron, 2
  • Copper wire, insulated with bare ends, 200 cm, 2 lengths
  • Clip for C-cores
  • Cell, 1.5 V in holder
  • Switch
  • Lead, 4 mm
  • Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

If a zinc chloride cell is used, it will polarise in 60 s or less and must be left overnight to recover.

If an alkaline manganese cell is used, there is a danger of the cell overheating with a risk of explosion: complete the circuit for 30 s or less.

If a re-chargeable cell (NiCd) is used, the wire will get very hot and the cell will be discharged in a few minutes: do the experiment as quickly as possible.

Read our standard health & safety guidance


It is possible to use a low-voltage power supply instead of the 1.5 V cells, but any ripple on the DC output can lead to confusion. There will be a deflection of the galvanometer even when the electromagnet is left switched on. Dry cells are therefore to be preferred.

Procedure

  1. Wind a coil of roughly 20 turns on one arm of a C-core.
  2. Connect the coil by long leads to a galvanometer.
  3. Wind 10 turns on one arm of the other C-core.
  4. Connect this coil to the 1.5 V cell and the switch.
  5. Observe the effect on the galvanometer of switching the current in the second coil alternately on and off.
  6. Try reversing the connections to the cell.
  7. Clip the two C-cores together.

Teaching Notes

  • Students will find that:
    • There is only a deflection on the galvanometer when the switch is turned on or off, i.e. when the current in the second coil (electromagnet) is changing;
    • There is no deflection on the galvanometer when the switch remains on, or remains off;
    • Reversing the connections to the battery results in an opposite deflection on the galvanometer.
  • In this experiment, students have effectively made a transformer. You could replace the cell with an AC supply and show that this gives an alternating output by connecting it up to a C.R.O.
  • The step up of the voltage from across the coil, connected to the cell, to the voltage, across the coil, connected to the galvanometer, shows the transformer in action. Voltmeters could be used to make measurements but it is more impressive to use two matched lamps. The lamp replacing the galvanometer is brighter than the lamp connected across the cell. If the C-cores are separated then the lamps will dim.

This experiment was safety-tested in July 2007

Up next

An electric motor used as a generator

V=-N(dΦ/dt)
Electricity and Magnetism

An electric motor used as a generator

Practical Activity for 14-16

Demonstration

You can generate an alternating current with a fractional horsepower motor.

Apparatus and Materials

Health & Safety and Technical Notes

For convenience, the motor should be mounted on a board, as shown, with 4 mm sockets allowing connections to the rotor and stator windings.

Read our standard health & safety guidance


Procedure

  1. Connect the armature (rotor) windings to the demonstration meter.
  2. Connect the field (stator) windings to the low-voltage supply.
  3. Set the supply to 2 V d.c. and switch on
  4. Turn the armature by rotating the pulley wheel on the shaft by hand.
  5. Reverse the direction of rotation to see any difference.
  6. Repeat without any voltage applied to the field terminals.

Teaching Notes

  • Only small dynamos have permanent magnets to make the magnetic field; large ones have electromagnets (whose coils are usually fed by a little of the dynamo's own output current).
  • The very large a.c. generators in power stations are called alternators. In them, the assembly of field coils rotates, driven by a turbine, and is called the rotor. The armature coils, in which the output voltage is generated, are held in a frame outside the rotor and remain stationary; this is the stator.
  • This arrangement is convenient for big machines because it does not need brushes and commutator or slip-rings to carry out the large output current. The spinning rotor's electromagnets are supplied with the small direct current they need from a small d.c. dynamo on the same spinning shaft as the big generator.
  • A dynamo spinning at constant speed with its field magnet kept at a constant strength produces a constant potential difference (the e.m.f.), like a well-behaved battery of cells. Even if there is no output current, a dynamo still produces an e.m.f. It is ready to drive a current. When you let it drive a current, by connecting something to its output terminals, the amount of current depends on the resistance of the thing which you connect (and the internal resistance of the dynamo coils).

This experiment was safety-tested in April 2006

  • A video showing a similar electromagnetic induction experiment:

Up next

Bicycle dynamo

V=-N(dΦ/dt)
Electricity and Magnetism

Bicycle dynamo

Practical Activity for 14-16

Demonstration

Showing how the brightness of a lamp increases when a dynamo powering it turns faster.

Apparatus and Materials

  • Bicycle dynamo assembly
  • Demonstration meter with dial (2.5–0–2.5 mA. DC)
  • Lamp, 1.5 V in lamp holder

Health & Safety and Technical Notes

Read our standard health & safety guidance


The dynamo assembly consists of a simple bicycle generator, mounted and geared so that it can be driven both at speed and slowly see the illustrations. For convenience, a lampholder may be fitted between the terminals. (Strictly this is not a dynamo because that term is reserved for a d.c. generator.)

Procedure

  1. Connect the output from the generator to the demonstration meter.
  2. Turn the handle with low speed gearing so that the deflection is clearly visible. Note the effect of increasing the speed of rotation. Note how, at higher speeds, the pointer merely vibrates over a very small range about the zero position, since the output is a.c.
  3. Turn the dynamo at high speed using the other gears (with the meter disconnected). The output will be sufficient to light a 1.5 V lamp connected across it.

Teaching Notes

  • A bicycle dynamo is one of the simplest of generators and is easily available. It also has the advantage that the armature/coil is stationary and the field moves relative to it, in accordance with standard practice in heavy engineering. The field is normally produced by an 8-pole circular magnet rotating between two coils, generating alternating voltages.
  • Turning the dynamo more quickly will increase the e.m.f.
  • The use of a demonstration meter is suggested so that it is clearly visible to the whole class. Afterwards, you may want to leave the dynamo at the side of the class for students to use for themselves. If you do this, confine this experiment merely to lighting the lamp (a sensitive meter may be damaged if the dynamo is turned very fast).

This experiment was safety-checked in April 2006

Up next

Bicycle dynamo and oscilloscope

Alternating Current
Electricity and Magnetism

Bicycle dynamo and oscilloscope

Practical Activity for 14-16

Class practical

Showing students that the e.m.f. (voltage) produced by a dynamo depends on the rate at which it turns.

Apparatus and Materials

  • Oscilloscope


  • Bicycle dynamo assembly
  • Lamp, 1.5 V in lamp holder

Health & Safety and Technical Notes

The dynamo assembly consists of a simple bicycle generator, mounted and geared so that it can be driven both at speed and slowly – see the illustration. For convenience, a lamp holder may be fitted between the terminals.

Read our standard health & safety guidance


Procedure

  1. Connect the output from the generator to the oscilloscope input (Y-plates). The time base should initially be switched off and there should be maximum gain on the Y-amplifier. The spot should be in the centre of the screen.
  2. Connect the lamp across the output of the dynamo, in parallel with the C.R.O.
  3. Turn the handle with low speed gearing so that the up and down motion of the spot is clearly visible.
  4. Switch on the time base at slow speed and centre the trace with the X-shift. The gain should still be at maximum. The dynamo is again driven at the slow speed, and the spot will be seen to generate a wave-like trace.
  5. Gradually speed up the time base. Cut down the gain on the oscilloscope to about 2 volts/cm and drive the dynamo at the high speed. With the time base set at 10 ms/cm, a roughly sinusoidal wave-form will be seen.

Teaching Notes

  • A bicycle dynamo is one of the simplest of generators and is easily available. It also has the advantage that the armature/coil is stationary and the field moves relative to it, in accordance with standard practice in heavy engineering. The field is normally produced by an 8-pole circular magnet rotating between two coils producing alternating voltages.
  • Turning the dynamo more quickly will increase the e.m.f.
  • A long-persistence screen would be an asset in this experiment, but is not essential. Alternatively, you could capture the trace using a datalogging system, and display it on a computer screen.
  • The wave form will not be sinusoidal; the bicycle dynamo was designed for efficiency and not for teaching purposes. Other generators can be found which give a more nearly sinusoidal wave form, but there is greater value here in using a generator as familiar as the bicycle dynamo.

This experiment was safety-tested in June 2007

  • A video showing how to use an oscilloscope:

Up next

Electromagnetic braking in a copper pipe

Lenz's Law
Electricity and Magnetism

Electromagnetic braking in a copper pipe

Practical Activity for 14-16

Demonstration

This demonstration always amazes 14-16 year olds, with whom it can be used to show some electromagnetic magic and demonstrate electromagnetic forces. Post-16 it can be used to illustrate electromagnetic induction and Lenz's law.

Apparatus and Materials

  • Copper pipe, 2 m, 16 mm diameter with smaller magnets
  • Copper pipe, 2 m, 22 mm diameter with larger magnets
  • Magnet, cylindrical rare earth (e.g. diameter 1 cm, 7 mm long)
  • Non-ferrous metal, pieces, of similar shape and size
  • Bucket or container of sand to cushion impact of magnet at floor level
  • Stopwatch
  • Data logger plus computer
  • Coil to act as sensor
  • Leads

Health & Safety and Technical Notes

Do not allow pupils to stand on the bench where they may fall over the tube.

Read our standard health & safety guidance


The photo comes from the work of a year 13 girl, Bethan James, who has been investigating the phenomenon.

Note that students should not work with bags on the bench (as shown in the photograph).

Procedure

  1. Clamp copper pipe vertically with sand bucket (or similar) underneath, so that the bottom of the pipe is about 20–30 cm above the sand.
  2. Drop non-ferrous metal from the top of the pipe as a control.
  3. Drop a magnet down the pipe and wait for wows. Repeat if required using a stopwatch to time the magnet.

Teaching Notes

  • The falling magnet induces eddy currents in the copper pipe (which acts effectively as a single one-turn coil). The magnetic field created by induced current opposes the change that caused it - this is Lenz's Law.
  • Does the magnet reach a terminal velocity? This is a question to investigate.
  • With the larger tube and cubical magnets, watch the magnet tumble as it falls down the pipe - lots of scope for student investigation.

This experiment was submitted by David Grace who teaches at Ysgol Y Preseli, Crymych, Pembrokeshire.

Up next

Faraday's law

Faraday's Law
Electricity and Magnetism

Faraday's law

Practical Activity for 14-16

Demonstration

Faraday's law of electromagnetic induction is a very important principle. Most of the electrical power in the world is generated by using this principle.

Apparatus and Materials

  • Ferromagnetic cylindrical rod: diameter 10 mm, length 10 cm
  • Insulated copper wire - SWG 30, 300 g
  • LED
  • Demountable transformer, used as a 'step-down' transformer with primary coil connected to mains, with turns ratio to produce 6-10 V across the secondary coil.

Health & Safety and Technical Notes

NB The primary coil must be designed for connection to the mains, e.g. using an IEC connector and mains lead.

Read our standard health & safety guidance


Procedure

  1. Wind the copper wire on the ferromagnetic rod to form a cylinder with the ferromagnetic rod as axis. Leave 0.5 cm of wire on either side of the rod. This is called the test coil.
  2. Remove the insulation from the two ends of the copper wire and connect an LED in series.
  3. Connect the primary coil of the demountable transformer to the AC mains.
  4. Hold the test coil (ferromagnetic rod with copper wire) in your hand and move the test coil close to the secondary coil, preferably along its axis. (A circular inductor coil will have its magnetic field along its axis.) The LED glows.
  5. If the primary coil is changed for a smaller one connected to the 20 V DC, the LED glows as the test coil is moved towards or away from the secondary coil.

Teaching Notes

  • When an alternating current flows through the secondary coil, it produces alternating magnetic field along its axis.
  • When the test coil is positioned correctly, the flux linking with it changes and a voltage is induced. The LED glows as the magnetic field oscillates due to an alternating supply current.
  • If a DC supply is used, the LED glows only when the test coil is moved.

This experiment was submitted by K.H. Raveesha, Head of Physics at the CMR Institute of Technology in India.

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